Evolved Universal Terrestrial Radio Access Network (UTRAN) is called enhanced UTRAN and E-UTRAN for short in Long-Term Evolution (LTE) research project.
In E-UTRAN, the structure of radio network controller (RNC)—NodeB is discarded and the E-UTRAN is completely composed of base station to simplify signaling procedure and reduce delay. In E-UTRAN, base stations communicate with each other by using Internet Protocol (IP) and logically connect with each other via X2 interfaces. Moreover, each of the base stations connects with an access gateway composed of a Mobility Management Entity (MME) and a User Plane Entity (UPE) via S1 interface.
E-UTRAN system is divided into a user plane and a control plane according to functions. The user plane mainly deals with service data of users and the control plane mainly deals with signaling.
In user plane protocol stack and control plane protocol stack, the functions of Packet Data Convergence Protocol (PDCP) sub-layer are located in an access gateway. The functions of the PDCP sub-layer mainly include header compression and decompression of IP data flows at the transmitting and receiving entity respectively, transmitting user data such as forwarding a PDCP Service Data Unit (PDCP-SDU) sent from Non Access Stratum (NAS) layer to Radio Link Control (RLC) layer.
In a conventional method, UE assisted network controlled handover is adopted during cell handover when UE switches between cells, i.e. the whole handover includes four processes of measurement-report-judgment-execution. When a source base station determines that the UE switches to a target cell according to measurement reports sent from the UE and a base station, the source base station directly communicates with a target base station via X2 interface to enable the target cell to prepare for resources. And then, the source base station notifies the UE of handover to the target cell. As the handover is finished, the target base station notifies the source base station of releasing radio resources which are held by the UE in the source cell. In addition, the source station transmits data that have not been transmitted to the target base station, and updates the relations among nodes in the user plane and nodes in the control plane.
Because there is no PDCP layer in a base station in the structure of a conventional protocol stack, both data and control signaling of a user are transmitted or retransmitted in RLC layer of a base station. When cell handover occurs and signaling handover procedure finishes, a target base station notifies MME and UPE of transferring a user plane path from an original base station to a new base station to avoid data loss in handover. The user plane path is established between a mobile gateway and a base station. And meanwhile, the source station transmits a data packet to the target base station via a user plane tunnel terminating at X2 interface in RLC layer. Such a mode is called data forwarding.
With the advance of evolution process, a viewpoint that the PDCP layer located in an access gateway is transferred to a base station has been proposed. The base station may directly compress and decompress the head of an IP data stream, and transmit the Packet Data Convergence Protocol-Service Data Unit (PDCP-SDU) to the RLC layer as the PDCP layer is transferred to the base station. When cell handover occurs, it is not applicable for forwarding or retransmitting both data and control signaling of a user in the RLC layer of a base station according to a convention method. Therefore, how to forward data during cell handover is an urgent problem to be settled at present.